Its the best biological filtration dollar for dollar.Using a combo of powerheads on some lifts and bubblers on the others you turn the entire gravel bed into a biological filter.2 qualifications:
1 regular gravel vaccing a must,so that rules out live plants in the substrate.
2 fish that do a lot of digging can reach the plates and disrupt flow
Have been using undergravel filters on unplanted show tanks for nearly 30 years,creates a more stable environment,less ammonia spike when adding fish,dont lose your biofilter if the canister breaks,seeds new filters rapidly.ideal for grow out tanks for juvies they dont get sucked into the filter.Some people worry about the buildup of inert material under the plates but with regular gravel vacs and good flowing heads it would take years to build up enough to require cleaning.Then you jusr run a thin hose down the lift tube and siphon.